As a general spark plug, there has been known a spark plug in which a noble metal tip (hereinafter referred to as tip) is welded to a front end of an electrode (center electrode or outside electrode) body so as to extend the life of the spark plug. In such a spark plug, a nickel alloy having nickel as its principal component is typically used for the electrode body. When the tip is simply welded with the electrode body having nickel as its principal component, there is, however, a fear that the tip separates from the electrode body in use so that the durability deteriorates. Therefore, a technique in which nickel is also contained in the tip so as to suppress the separation of the tip is disclosed in the following Patent Document 1. In addition, a technique in which the tip is designed to have a two-layer structure of a low-nickel-content portion and a high-nickel-content portion, and the high-nickel-content portion is welded with an electrode body, is disclosed in the following Patent Document 2.
Patent Document 1: Japanese Patent Laid-Open No. 26480/1983
Patent Document 2: Japanese Patent Laid-Open No. 135081/1986